A. Field of the Invention
This invention relates generally to amplifiers and, more particularly, to balanced direct coupled power amplifiers used for powering loud speaker systems.
B. Description of the Prior Art
Transistorized power amplifiers suitable for driving loud speakers are known. A variety of configurations may be used for such amplifiers, such as complementary symmetry transistors (one PNP and one NPN transistor or one P-channel and one N-channel FET) connected as push-pull emitter followers or push-pull common emitter stages; or like conductivity type transistors (both PNP or NPN transistors or both P-channel or N-channel FETS) may be used in a push-pull configuration, with one of the transistors being operated as an emitter follower and the other of the transistors being operated as a common emitter stage.
While these systems provide a way to power a loud speaker system, several problems are associated with each of the prior art systems. For example, when a common emitter stage is used in a push-pull configuration in conjunction with an emitter follower stage, the gain characteristics of the two stages forming the push-pull circuit are inherently unequal. Consequently, feedback circuitry must be employed to compensate for such differences, and to reduce the distortion caused by such differences. Such feedback circuitry usually takes the form of an external feedback loop, such as collector to base feedback. Similarly, feedback must be employed when two complementary symmetry stages are connected in a common emitter configuration to reduce distortion inherent in common emitter stages. Such feedback increases the complexity of the amplifier, and reduces its stability.
Another problem associated with the prior art transistor amplifiers arises in the coupling of the loud speaker system to the output of the amplifier. Direct coupling to the amplifier is advantageous from a simplicity and low-frequency response standpoint, but such direct coupling results in a direct current path through the loud speaker which can cause excessive direct current to flow through the voice coil of the loud speaker. Such direct current can adversely bias or saturate the voice coil of the loud speaker and result in distortion. Consequently, in order to avoid such a direct current flow, the prior art amplifiers utilize coupling capacitors in series with the loud speaker to block such direct current flow, or employ a split power supply that maintains the potential difference between the terminals of the amplifier to which the loud speaker is connected at substantially the same potential.
The use of capacitor coupling has the disadvantage that unless a very large capacitor is used, an attenuation of low frequencies results. Furthermore, the use of a capacitor in series with the loud speaker can produce a loud pop in the speaker during the time the capacitor is being charged after the unit is turned on. The use of a split power supply requires precise balancing of the amplifier, and usually requires a balance control to maintain the output terminals of the amplifier at the same potential. Furthermore, failure of a component in the output or balancing circuits of the amplifier can cause substantial direct current to flow through the speaker. In addition both solutions significantly increase the cost of the amplifier.